This invention relates to a method and apparatus for transmitting data frames in frequency domain data transmission systems, and particularly for implementing forward error correction in orthogonal frequency division multiplexed (OFDM) data transmission systems.
It is known to transport telecommunications signals over a medium in a plurality of separate frequency channels at the same time. This can be achieved using a frequency domain modulation scheme such as OFDM or coded orthogonal frequency division multiplexing (COFDM) and is desirable in transmission media which carry other signals in predetermined frequency bands or suffer from noise in particular frequency bands. Examples of such media are the use of powerlines (PL) for transporting telecommunications data to and from subscribers, cable television (CATV) systems and fixed wireless access (FWA) systems.
Use of OFDM for signal transmission in PL media is known from U.S. Ser. No. 09/419,209, which is incorporated herein by reference. U.S. Ser. No. 09/419,209 also describes the use of OFDM for signal transmission in more than one frequency band, as follows.
One of the problems with using power lines as a communications medium is that they are subject to noise and interference, for example due to cables picking up radio signals such as broadcast AM radio signals and amateur radio band transmissions, or electrical noise from electrical equipment coupled to the power lines. Noise propagates along the power lines and can corrupt communications signals.
CATV and FWA suffer similar problems, though not necessarily from the same sources. For example, in CATV noise can result from ingress at the consumer connection and inter-modulation products from the TV carriers.
FIG. 1 shows typical background noise on an underground power line across the frequency band 0-10 MHz. It is known to be advantageous to transmit communications signals within the frequency bands 2.2-3.5 MHz (PLT1) and 4.2-5.8 Mz (PLT2) because these bands fall between the medium wave and short wave bands used for broadcast radio transmissions and avoid the radio amateur band at 3.5-3.8 MHz. There is a reduced level of background noise in these bands and the radiation of power line communications signals in this frequency band causes minimum interference with radio receiver equipment at subscriber premises. Other frequency bands in the range of, for example, 2-30 MHz can be used although it is preferred to use the lower frequencies because attenuation over the distribution cables is lower.
Upstream and downstream transmissions preferably share a common frequency band with the upstream and downstream transmissions occupying different times.
The use of OFDM provides flexibility to fit into non-uniform and non-contiguous frequency allocations, while maintaining reasonable spectral efficiency. This results from the intrinsic nature of OFDM which is composed of a large number of simultaneously transmitted sub-carriers which are staggered in frequency each individually occupying a low bandwidth, as illustrated in FIG. 2. The scheme""s flexibility comes about from the ability of designate which sub-carriers are to be activated and which are not. Regarding spectral efficiency, the signal composition results in an intrinsic spectrum fall-off outside of the active bandwidth commensurate with the bandwidth of each sub-carrier rather than with the total spectrum width. Thus relatively low excess bandwidths can be achieved.
Therefore, the spectral attributes of OFDM represent a major advantage in favor of its selection for use in power line telecommunication systems.
A further difficulty of transmission over certain media in more than one frequency band is frequency selective fading, particularly if the frequency bands are non-contiguous. An OFDM transmission system is again suited to such systems because of the possibility for adaptively selecting which portions of the frequency spectrum are to be used, enabling the avoidance not only of mutual interference but also of areas of poor transmission capability in the frequency spectrum which may arise from time to time due to frequency selective fading.
A problem then arises as to how to encode framed data for transmission over a variable bandwidth in an OFDM system requiring use of a constant OFDM symbol length because, as the available bandwidth varies, so does the total capacity of each symbol. A further problem then arises as to how to apply effective error detection or correction to frames of variable capacity.
An object of the invention is to provide an efficient method and apparatus for handling data for transmission over a variable bandwidth transmission channel or a transmission channel in which transmissions are carried in two or more non-contiguous channel portions.
A further object of the invention is to provide effective error correction for a transmission over a variable bandwidth transmission channel or a transmission channel in which transmissions are carried in two or more non-contiguous channel portions.
The invention provides a method and an apparatus for transmitting a data frame between a transmitter and a receiver in a frequency domain data transmission system using a transmission channel from which one or more channel portions are selected, by adaptive or preemptive selection or by allocation, for data transmission such that the channel contains one or more unused channel portions, comprising the steps of;
inserting data bits into a payload portion of the data frame;
padding with a predetermined bit sequence a portion of the payload;
applying error correction to the data bits and the padding bits and inserting error detection bits into an error detection portion of the frame to form a data frame of a predetermined length;
mapping the data frame into the transmission channel such that the payload and the error detection bits are mapped to and transmitted in the one or more selected channel portions and the padding bits are mapped to the one or more unused channel portions and are not transmitted;
receiving the data bits and the error detection bits from the one or more selected channel portions;
reinserting the padding bit sequence to restore the predetermined-length frame, subject to any transmission errors;
applying error correction using the error detection bits to correct the transmission errors in the data bits, where possible; and
extracting the data bits.
In further aspects the invention provides a transmission method and a reception method for carrying out the method above, and a transmitter and a receiver for implementing these methods, as defined in the appended claims.
In a further aspect, the invention provides a reduced-length data frame, comprising the payload and the error detection bits as transmitted and received by the method and apparatus of the invention.
Advantageously, the data padding may comprise zero padding, in the form of a sequence of zero bits.
The invention may operate advantageously with any block coding error correction scheme, such as Reed-Solomon coding.
In a transmission system embodying the invention, after the step of applying error correction and before transmission, a padded data frame may undergo conventional techniques such as interleaving, inner coding and energy dispersal. Such techniques may rearrange the bits within a frame but are predictable, so that they can be taken into account in the step of mapping the payload and the error-detection bits to the adaptively-selected channel portion or portions.
The transmission channel into which the invention maps a data frame is a contiguous frequency band. Within that band, one or more channel portions are adaptively selected to carry the payload and error detection portion of the data frame; padding bits within the payload map into any unused channel portions(s). In various aspect the invention may therefore find a variety of applications. For example, if two or more non-contiguous channel portions, or sub-channels, are available for data transmission (as in the PL application described above), those non-contiguous channel portions may be considered as being within a larger, contiguous channel and each frame padded such that the payload and error detection bits map into the non-contiguous channel portions and the padding bits map into the unused frequency bands between them.
In a further aspect of the invention, the transmission channel may be sounded, or monitored, during use and the channel portion or portions for future transmission adaptively selected in real time in response to channel conditions. Thus, for example, if a channel portion generates a high transmission error rate it may be selected not to be used. A portion of the payload of corresponding size in future frames may then be padded and mapped into this unused channel portion.
These aspects of the invention may also be combined, so that a system initially set up using two or more non-contiguous channel portions may sound, or monitor, each channel portion and stop using one or more of them if channel conditions in that channel portion deteriorate over time. This embodiment of the invention may find particular application in a PL system, for example, where non-contiguous channel portions are used in a noisy environment.
In summary, a preferred embodiment of the invention may incorporate the following features. In an adaptive or preemptive transmission system, both the transmitter and the receiver of a data frame may advantageously know in advance which parts of the frequency spectrum will be used for transmitting a frame, and thus the total available bandwidth. With this knowledge when the frame is assembled for transmission, padding may be inserted into a portion of the frame corresponding in size to the capacity of a frequency band or bands (channel portion or portions) within the transmission channel which are not to be used for transmission due to frequency selective fading. FEC (forward error correction) block coding can then be applied to the payload of the padded frame, and the frame mapped into the frequency domain for transmission such that the payload and the error detection bits map into the adaptively or preemptively selected channel portion or portions, and the padding bits map into the unused channel portion or portions (which have not been selected for transmission). It should be noted that in this way the frame, which is of predetermined length, maps into the whole of the transmission channel. A constant symbol length is therefore retained despite the adaptively-selected bandwidth being less than the contiguous channel bandwidth, while the padding bits are not actually transmitted. The receiver may then impose the same padding on the received frame before FEC decoding. This may advantageously automatically increase the coding power, or error correction capability, of the FEC coding system if the total available bandwidth reduces due, for example, to frequency selective fading. This effect is preferably achieved while retaining a chosen predetermined FEC coding overhead, which ensures efficient FEC block coding.
Therefore, according to a preferred embodiment of the invention, as the adaptive frequency allocation process reduces frequency (bandwidth) allocation, the number of data bits in each data frame, or symbol, is reduced, but the chosen FEC coding overhead remains the same so that the effective error correction capability of the FEC coding increases.
The invention may therefore advantageously allow improved flexibility in the adaptive frequency allocation process and also provide an adaptive coding power, or error detection; capability, which tracks channel conditions by providing increased coding power as channel conditions deteriorate or as channel bandwidth is otherwise reduced.
In an alternative preferred embodiment, the relative numbers of data bits and error detection bits in a frame may be adaptively varied in response to the available bandwidth of the adaptively-selected channel portion(s) in order, for example, to maintain the same error correction capability under all channel conditions. This advantageously maximises the data transmission rate under all channel conditions without reducing error correction capability.
It can be seen that the invention addresses a problem arising from the variation in data load in each symbol in an OFDM system in which transmission bandwidth is variable. Block coding, such as Reed Solomon coding, has been found to be optimum for forward error correction (FEC) in many instances, such as in PL systems, and variable data load per OFDM or COFDM symbol places constraints on such block codes. These constraints are alleviated by the invention.
To illustrate this advantage of the invention, an alternative method of implementing block coding in conditions requiring a variable data load in each symbol, to match the variable bandwidth of the channel, might be to divide the data stream into small fixed-length sequences and to constrain the adaptive process to the size of these sequences. In such a scheme, if the channel bandwidth changes due to fading of a channel portion, or sub-channel, the bandwidth reduction would be accommodated by transmitting fewer fixed-length sequences rather than by transmitting symbols (larger than the sequences) with variable data load. In such a scheme, block coding could be applied to each small fixed-length sequence in conventional manner. However, this approach has a number of disadvantages. First, small code block lengths lead to high overheads, both in decoder processing and data overhead. Second, the error correction capability is fixed for a given code overhead per block, and is therefore disadvantageously inefficient. Third, the use of fixed-length sequences disadvantageously constrains the adaptive frequency selection process.
The invention has been described mainly in the context of channel bandwidth varying due to the impact of noise or interference, but it has many other applications, including the following.
In different environments or applications, different channel bandwidths may be available for transmission, for example due to different regulatory spectrum restrictions in different regions. In a preferred embodiment, the invention may provided a fixed hardware implementation, using predetermined frame and symbol lengths and predetermined error detection, which can advantageously accommodate such different bandwidth allocations. Preferably, only software modifications may be required to allow the hardware to operate in such different applications. The cost advantages of a fixed hardware implemention may then be achieved by the invention even if the bandwidth allocations are not affected by variable channel conditions. The adaptive channel portion selection step of the invention may then only be performed once, when the hardware implementation is set up and the channel portion(s) for transmission allocated, and not necessarily in real time during operation of a transmitter and receiver to follow varying channel conditions.
In a further embodiment, the invention may advantageously enable efficient adaptive sharing of bandwidth between two of more users. If a controller adaptively allocates bandwidth in a channel between two or more transmitters embodying the invention, and informs the transmitters and corresponding receivers of the adaptive allocation of channel portions to each within the channel, the transmitters may advantageously adapt the number of data bits in each frame they transmit in order to map their transmitted frames onto their allocated channel portion or portions.
The applications of the invention described herein are exemplary applications and do not form an exhaustive list. Further applications and embodiments of the invention will be readily apparent to the skilled person in the light of the present disclosure.